1. Closed Loop Hydraulic Systems.
Hydraulic systems typically include the following elements: a low-pressure reservoir; a pump; a high-pressure reservoir; an actuator; and various related lines, hoses, and fittings. A fluid is drawn from the low-pressure reservoir by the pump. The fluid is then stored in the high-pressure reservoir, also referred to as an accumulator, until the fluid is needed to motivate the actuator. Once the fluid has been used by the actuator, the fluid is returned to the low-pressure reservoir.
For any hydraulic systems application, minimizing power losses in the high pressure portion of the hydraulic system is critically important. It is known to use pipes and hoses to transport pressurized fluid between the various high pressure components. However, the pipes and hoses of the hydraulic system account for a large fraction of the lost power. In addition, the pipes and hoses are typically expensive, require precision fittings, are subject to failure, and are prone to leaking.
2. High Pressure Accumulator Designs in the State of the Art.
In the state of the art, there are three basic configurations for hydraulic accumulators, namely: spring type; bladder type; and piston type.
Spring type accumulators are typically limited to applications with small fluid volumes due to the size, cost, mass, and spring rates of the springs.
Bladder type accumulators typically suffer from high gas permeation rates and poor reliability. Some success has been achieved by replacing the elastic bladder with a flexible metallic or metallic-coated bellows structure, for example, as disclosed in U.S. Pat. No. 5,771,936 to Sasaki et al. and U.S. Pat. No. 6,478,051 to Drumm et al., the entire disclosures of which are hereby incorporated herein by reference.
Piston accumulators typically employ two chambers housed within a cylindrical pressure vessel, in which the hydraulic fluid is separated from the compressed gas by means of a piston which seals against the inner walls of the pressure vessel. The piston is also free to move longitudinally as fluid enters and leaves and the gas compresses and expands.
3. Safety Concerns Inherent with the State of the Art.
The high pressure components of the hydraulic system, and in particular hoses, pose a potential hazard to those nearby due to the energy contained therein. While the likelihood of a catastrophic failure of a pump or motor is relatively minor, hoses can easily rupture, especially if compromised in any manner.
Posing a significant potential hazard are the pipes and hoses in high pressure fluid communication with the accumulator, due to the energy they store. While one might envision a sudden rupture as the most potentially dangerous failure, a small hole also poses a significant threat as the escaping stream of the fluid may easily harm or damage an individual, for example, sever a human limb of a passerby.
As these concerns are well understood, designers employ large safety factors and/or configure the system in such a manner that the pressurized components are positioned far from possible human contact. For example, accumulators are typically designed with a safety factor of four, resulting in a device that is approximately four times heavier than theoretically needed.
Recent technological developments, such as the hydraulic hybrid power train, challenge both of these approaches. Since these power trains are intended for vehicular use, and are specifically employed because of the improvement in fuel economy they offer, they cannot be any heavier than necessary, nor can they be positioned remote from the vehicle's occupants.
Another method for addressing the problem of high pressure pipes and hose includes, for example, U.S. Patent Appl. Pub. No. 2007/0084516 to Rose, the entire disclosure of which is hereby incorporated herein by reference. Rose teaches the placement of the high-pressure reservoir within an outer casing. The outer casing both contains fluid and is itself pressurized, thereby insufficiently addressing the safety issue. Other art in which the outermost casing is a pressure vessel, includes U.S. Pat. No. 7,108,016 to Moskalik et al., U.S. Pat. No. 7,013,923 to Suzuki, U.S. Pat. No. 6,923,223 to Trzmiel et al., U.S. Pat. No. 6,076,557 to Carney, and U.S. Pat. No. 5,709,248 to Goloff, the entire disclosures of which are hereby incorporated herein by reference.
U.S. Pat. No. 3,907,001 to Vanderlaan and Boyle describes a combination accumulator reservoir device in a single package. The high pressure reservoir is fully exposed to the external environment and is vulnerable to the same shortcomings as the other art described hereinabove.
4. Thermal Inefficiencies with the Present State of the Art.
Another shortcoming of present piston type accumulators is thermal inefficiency due to the uncontrolled manner in which the energy flow associated with the compression and expansion of the gas is managed. When fluid is forced into the accumulator, the gas contracts and, in so doing, heats up. Since the temperature of the heated gas is typically greater than that of the surrounding, ambient environment, energy starts to flow from the gas to the ambient environment, thereby removing energy from the hydraulic system. When fluid is removed from the accumulator, the gas expands and its temperature decreases, though typically not below that of the ambient environment. Thus, not all of the energy that is stored in the gas can be extracted due to thermal interactions with the surrounding, ambient environment.
There is a continuing need for a high pressure hydraulic system that minimizes power loss and maximizes the safety of the hydraulic system. Desirably, the hydraulic system is comparatively inexpensive relative to prior art systems, and militates against leaking and failure within the hydraulic system.